1. Field of the Invention
This invention relates generally to thermal actuators and, more particularly, to thermally actuated switches that have a rapid snap action that rapidly opens and closes the contacts to minimize arcing and contact resistance to thereby prolong the life of the switch while accurately maintaining the switching temperatures.
2. Description of the Prior Art of a thermal switch are known. Typically, such actuators employ a bimetallic member, such as, for example, a disc that has a high temperature stable state and a low temperature stable state and snaps with a snap action from the low temperature stable state to the high temperature stable state upon heating and returns to the low temperature stable state upon cooling. In such devices, the bimetallic member snaps to its high temperature stable state at a predetermined temperature known as the "upper set point" and returns to its low temperature stable state at a lower temperature known as the "lower set point". The temperature difference between the upper and lower set points is known as the temperature differential. Other thermal actuators utilize a plurality of bimetallic members, such as, for example, two discs as disclosed in copending patent application U.S. Ser. No. 07/232,244 filed by the same inventor named in the present application on Nov. 18, 1988 and assigned to the same assignee as the assignee of the present invention, and incorporated herein by reference. In the aforementioned application, the thermal actuator utilizes two mechanically coupled bimetallic discs having different upper and lower set points selected so that one of the discs controls the transition from a first to a second state and the other one of the discs controls the transition from the second to the first state. The set points are selected so that the upper set point of the second bimetallic member is between the upper and lower set points of the first bimetallic member, and the lower set point of the second bimetallic member is lower than the lower set point of the first bimetallic member. The members are mechanically coupled to form a bistable thermal actuator having a high temperature stable state determined by the upper set point of the second bimetallic member and a low temperature stable state defined by the lower set point of the first bimetallic member. The use of two bimetallic members instead of a single bimetallic member provides better control of the upper and lower transition points and a better snap action than can be achieved by a single bimetallic member.
When a thermal actuator is utilized to actuate a device, such as a resiliently biased switch or other resiliently biased device, the device being actuated exerts a force on the actuator in a direction tending to prevent the change of state of the bimetallic member when the change of state is in a direction opposing the biasing force exerted by the device being actuated. Conversely, the biasing force of the actuated device aids the transition of the bimetallic member when the direction of the transition is in the direction of the biasing force. Thus, while the biasing force of the actuated device may aid the transition in the latter case, the opposition of the biasing force to the transition in the former case reduces the speed or quality of the snap action, particularly after the actuator has been cycled a large number of times.